feat: better report message

[tmzane]

The new form is:

`User` should be annotated with the `json` tag as it is passed to `json.Marshal` at <location>

[codecov-commenter]

Codecov Report

Merging #27 (98bf0aa) into main (c99a52e) will decrease coverage by 1.06%.
The diff coverage is 84.00%.

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@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   89.94%   88.88%   -1.06%     
==========================================
  Files           2        2              
  Lines         179      189      +10     
==========================================
+ Hits          161      168       +7     
- Misses         12       14       +2     
- Partials        6        7       +1     

Impacted Files	Coverage Δ
utils.go	77.77% <63.63%> (-8.59%)	⬇️
musttag.go	90.74% <100.00%> (+0.29%)	⬆️

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